(a) Field of the Invention
This invention relates to an optical information reading apparatus to be used in a video disk system in which cross-talks are decreased and S/N of read signals is improved.
(b) Description of the Prior Art
There is known an optical information reading apparatus wherein a beam radiated from a light source and collected is projected onto a recording medium having a track including recorded information and a plurality of detectors are arranged so as to receive a light pencil modulated by the recorded information. There is known, for example, such optical information reading apparatus wherein information is read by focusing a reading light spot through an objective on an information track arranged spirally or concentrically circularly on a recording medium. There is a recording medium having an information track and called, for example, a video disk, audio disk or data disk. In such disk, coded video signals, audio signals or data signals are recorded in the information track as optical information which can be represented by an optical transmission characteristic, reflection characteristic or phase characteristic. The information recorded in such disk is read by focusing through an objective on the information track a laser light radiated from a laser light source while the disk is rotated at a high speed and detecting a transmitted light or reflected light modulated by the information track. One of the features of such recording medium is that the information recording density is very high. Therefore, the width of each information track is very narrow and the spacing between the successive information tracks is also very narrow. In order to accurately read the original information from the information track thus narrow in both width and pitch, it is necessary to always accurately project on the track of the disk a beam spot focused by the objective. However, as the relative positions of the disk and objective fluctuate, the spot can not always be held on the track. Therefore, such optical reading apparatus is controlled by a servomechanism whereby the position lag of the beam spot from the information track is detected and the spot is displaced in the direction at right angles with the information track and the optical axis of the objective and the optical axis direction of the objective on the basis of this position lag signal.
FIG. 1 shows the optical system of the above described disk reading apparatus. A disk 1 is rotated at a speed, for example, of 1800 revolutions per minute by a spindle 2. A concentric circular or spiral track 3 is recorded on the disk 1. Such light as a laser light radiated out of a light source 4 is focused by a lens 5, .lambda./4 plate A, half mirror 6, reflecting mirror 7 and objective 8 and is projected as a spot on the track 3 of the disk 1. The light reflected by the disk 1 is collected by the objective 8 and the light reflected by the reflecting mirror 7 and half mirror 6 is made to enter the light receiving device 10 through a lens 9. In the apparatus shown in FIG. 1, the light receiving device 10 is positioned in the far field zone of the information track 3. That is to say, the light receiving device 10 is arranged at the position well away from the image of the pit structure formed by the objective 8 so that the diffracted beams of various orders formed by the pit structure of the information track can be detected as separated.
Further, there is also known an optical information reading apparatus wherein the light receiving device 10 is arranged at the position in which the image of the pit is formed by the objective 8.
Among the factors deteriorating the picture quality in such optical video disk system are S/N of a read signal and a cross-talk. In case information is being recorded with a spatially high frequency as in a portion near the center of a disk or an information track portion smaller in the diameter, the signal to be read will be so small that the former will be particularly a problem. Further, it is known that particularly, in the case of reproducing information in a CLV type video disk system, the latter will have a great influence on the picture quality.